Various control systems have been developed up to this time in order to obtain best revolution speeds of an engine in various operation modes thereof. To adjust ignition timing of an engine is applied as one of these control systems.
In an ignition timing adjusting apparatus of an electric control type, the adjustment is achieved by previously obtaining the most suitable ignition timing corresponding to various conditions of an engine, for example, fuel injection pulse width and engine revolution speed according to an experience etc. thereof, and by storing data corresponding to the detected results in a memory element such as ROM (read-only memory). And, in an actual operation of an engine, the adjustment of ignition timing is effected by reading out the ignition timing data according to fuel injection pulse width and engine revolution speed in the actual conditions of an engine.
When an accelerator pedal is pushed, a large amount of combustion air is supplied to the engine, and then the combustion fuel is increased to accelerate engine speed. It has been known that, in an acceleration phase, the fluctuation in the engine speed is involved in order that the engine revolution is not rised smoothly. There has been a problem of an uncomfatable riding feeling that the fluctuation in the engine speed is uncomfatable as a bodily sensation so that it is wobbly felt in operation of engine (hereinafter referred to a wobbly vibration).
The wobbly vibration is caused as the fluctuation of an engine revolution speed by unstable one. The prevention of the wobbly vibration can be effected by controlling the engine speed so as to suppress the engine speed when it is increased and increase the engine speed when it begins to be decreased.
Japanese Patent Laid-Open No. 61-157767 discloses a method for preventing such a wobbly vibration, which method is based on that an engine torque can be controlled by advancing or delaying the ignition timing of engine. Accordingly, when engine speed become low in causing the wobbly vibration, engine torque is increased by advancing ignition timing, while, when high in causing the wobbly vibration, engine torque is decreased by delaying ignition timing.
Further, Japanese Patent Application No. 61-117312 filed by the present applicant discloses a method where a basic ignition timing is compensated by ignition timing compensation magnitude determined on the basis of an engine speed fluctuation during a predetermined period of engine operation. Since this method compensates the ignition timing in consideration of the engine speed fluctuation, it has been possible to effectively prevent the wobbly vibration.
At that time, engine torque is considerably fluctuated when the ignition timing is compensated in such case that a throttle valve is unnecessarily opened, while engine torque is not fluctuated considerably by compensating the ignition timing in case of a partial engine load. Thus, it is seen that the torque value determined by the engine load is different from that determined by the compensation magnitude of ignition timing. Therefore, the wobbly vibration could not be effectively prevented by compensating the basic ignition timing only on the basis of a revolution speed fluctuation of the engine during a predetermined period of the engine operation, because its compensation is not fully considered as to the engine load.